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- Frost, Anna E., 1988-
(författare)
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In the Air Gap of Linear Generators for Wave Power
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wave power conversion is one type of renewable electricity generation. Within wave power, there are many different concepts, whereof some utilizes linear generators for converting the energy in the ocean waves into electricity. A linear generator consists of a translator, which is moving and have magnets of alternating polarity, and a stator, which have conductors sur-rounded by laminated steel. Between the translator and stator is an air gap, which is only a few millimeters wide. All linear generators for wave power, to the author’s knowledge, are permanent-magnet synchronous generators. This thesis looks into the forces and power flow in the air gap of linear generators for wave power, with the purpose of improving their future performance. The studies have focused on permanent magnet synchronous generators for wave power, but several of the results should also be applicable for other applications of linear elec-trical machines.Depending on the design of the linear generators, the translator can move so long that it only partially overlap the stator. This is common among several different wave power concepts with linear generators. When the stator is only partially overlapped by the stator it is denoted as partial stator overlap. It is studied how partial stator overlap affects the generated electric-ity, the absorbed energy, and the tangential and normal force in the air gap. The generated electricity and absorbed energy of a linear generator are quadratically dependent on the partial stator-translator overlap is shown through Faraday’s law and simulations. Experimental data showed that the absorbed energy is both linearly and quadratic depending on partial stator over-lap, where the linear dependence is at least partially due to frictional losses. Simulated results confirm that voltage is linearly dependent on partial stator overlap, which means quadratic de-pendence between generated electric and partial stator overlap. The simulated forces showed a linear dependence.Decades ago, the Poynting vector was used to derive an expression for the power flow in the air gap of rotating electrical machines. In this thesis the equivalent expressions for both flat and tubular linear electrical machines were derived. The analytical results were also compared with results from simulations. Both the analytical expressions and simulations showed that tubular and flat linear electrical machines have slightly different behavior.
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| 2. |
- Lejerskog, Erik, 1977-
(författare)
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Theoretical and Experimental Analysis of Operational Wave Energy Converters
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis studies wave energy converters developed at Uppsala University. The wave energy converters are of point absorbing type with direct driven linear generators. The aim has been to study generator design with closed stator slots as well as offshore experimental studies.By closing the stator slots, the harmonic content in the magnetic flux density is reduced and as a result the cogging forces in the generator are reduced as well. By reducing these forces, the noise and vibrations from the generator can be lowered. The studies have shown a significant reduction in the cogging forces in the generator. Moreover, by closing the slots, the magnetic flux finds a short-cut through the closed slots and will lower the magnetic flux linking the windings.The experimental studies have focused on the motion of the translator. The weight of the translator has a significant impact on the power absorption, especially in the downward motion. Two different experiments have been studied with two different translator weights. The results show that with a higher translator weight the power absorption is more evenly produced between the upward and downward motion as was expected from the simulation models. Furthermore, studies on the influence of the changing active area have been conducted which show some benefits with a changing active area during the downward motion. The experimental results also indicate snatch-loads for the wave energy converter with a lower translator weight.Within this thesis results from a comparative study between two WECs with almost identical properties have been presented. The generators electrical properties and the buoy volumes are the same, but with different buoy heights and diameters. Moreover, experimental studies including the conversion from AC to DC have been achieved.The work in this thesis is part of a larger wave power project at Uppsala University. The project studies the whole process from the energy absorption from the waves to the connection to the electrical grid. The project has a test-site at the west coast of Sweden near the town of Lysekil, where wave energy systems have been studied since 2004.
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